Fracastorius is the lava-flooded remnant of an ancient lunar impact crater located at the southern edge of Mare Nectaris.

The northern wall of this crater is missing, with only mounds appearing in the lunar mare to mark the outline.

Fracastorius has no central peak, but a long, slender rille runs across the middle of the floor in a generally east–west direction.

The crater commemorates the Italian scholar, astronomer and poet Girolamo Fracastoro, "Fracastorius" (1478‑1553).

[1] By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Fracastorius.